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According to https://chromestatus.com/feature/5188237101891584, this feature was only in Origin Trial from Chrome 133 to 135. Can you clarify if these features landed unconditionally in Chrome Stable?

They’re in current Chrome (143) behind the experimental web platform features flag. I’m not certain, but my understanding is that the feature was never backed out.

There’s also experimental implementation in Safari as of 26. I added in the flags information for both Chrome and Safari.

A Firefox implementation is also currently being reviewed, though it hasn’t landed yet, so don’t have a version number to add yet.

It looks like these landed unconditionally in Firefox 144.

This is behind a runtime flag in Firefox: dom.shadowdom.referenceTarget.enabled
In the patch from the bug you link you can see it's behind the flag: mozilla-firefox/firefox@e5cf85c1e4ee#diff-3e8d687f469c6af2120accb01e7664b85d57467e9b1aa0d65fd9ad916c1c5a21

Also you can check this test https://wpt.live/shadow-dom/reference-target/tentative/reference-target-basics.html with the flag enabled and disabled, to see what happens.

The Firefox prototype implementation for this feature has been completed a few days ago, so people can start playing with it there (the same than chromium and webkit). All of them behind a runtime flag.
The Firefox version would be 149 if you see the latest bugs closed related to this feature (e.g. https://bugzilla.mozilla.org/show_bug.cgi?id=2008915).